Little is understood about the genetic modifiers that are responsible for a wide range of complications that afflict autosomal recessive polycystic kidney disease (ARPKD) patients. Understanding what these genetic modifiers are, how they disrupt organ function, and how they aggravate ARPKD, will lead to more effective and tailored therapies aimed at ameliorating patient suffering. Experimental results strongly suggest that reduced proteoglycans (PGs) are modifiers of PKD lesion severity. Therefore, the broad long-term goal of our research group is to identify the responsible PGs and determine their role in the pathogenesis of PKD with the intent of developing targeted treatments that reduce lesion severity. In pursuit of this goal, our investigations have identified several PGs in liver that when lacking glycosaminoglycans are candidates for cyst development. Based on extensive preliminary data, this application's central hypothesis is that reduced PG levels are important modifiers of cyst development and can augment ARPKD cyst development and progression mediated by increased canonical Wnt/?-catenin pathway activity. The overall objective of this application, as a first step towards our long-term goal, is to test this hypothesis with the following aims: 1) To determine the impact of reduced PG on the development of liver and renal cysts in an ARPKD mouse model with decreased FC activity and loss of XylT2 activity. Cyst development, severity, and lesional ?-catenin levels in these mice will be measured using comprehensive histological methods and magnetic resonance imaging; 2) To determine the role that PGs have in renal cyst development by creating adult Xylt2-/- mice that have significantly reduced renal PGs. Renal function, cystic development, and glycosaminoglycan content will be assessed in the resultant mice and compared to controls; 3) To determine the importance of recently identified candidate PGs in the regulation of Wnt/?- catenin signaling in cultured biliary tracts and in isolated biliary epithelial cells. Wild type and Xylt2-/- cells and biliary tracts will be cultured under conditions of increased Wnt/?-catenin activation in the presence of core protein with and without glycosaminoglycans. Wnt/?-catenin signaling will be assessed by luciferase reporter assays and by immunohistochemistry for pathway intermediates and correlated with cellular proliferation and cyst development. The rationale of the proposed research is based on the assumption that understanding the molecular mechanism of ARPKD lesion modifiers will translate into fundamental improvements in understanding PKD lesion development. Thus the proposed research is relevant to PA-07- 293 in regards to defining the function of molecules and pathways that cause, aggravate or ameliorate PKD and identifying in rodent models...specific genes that modify the clinical phenotype. The proposed research is significant because the identification of genetic modifiers and understanding their impact on clinical variation is expected to advance individualized ARPKD patient treatment and prognostication.